What will happen if you run a window air conditioner inside?

A common misconception is that A/C units generate cold. However, Air conditioning works as a heat exchange system, so it removes heat from indoors and pushes it outside. By running a window A/C unit inside you are just moving the heat from one side of the a/c unit to the other, and since it is not a 100% efficient machine, you will in fact be generating heat and create an overall rise in temperature.